1. Field of the Invention
The present invention relates to particle beam irradiation equipment. More particularly, the present invention relates to particle beam irradiation equipment, which is suitably applied to particle beam treatment for irradiating a charged particle beam of, e.g., proton ions or carbon ions, to a tumor for treatment, material irradiation equipment for irradiating a charged particle beam to materials, food irradiation equipment for irradiating a charged particle beam to foods, and radio isotope producing equipment utilizing a charged particle beam, and also relates to a method of adjusting irradiation field producing equipment used in the particle beam irradiation equipment.
2. Description of the Related Art
Known particle beam treatment equipment comprises charged particle beam generation equipment, an ion beam transport system, and rotating irradiation equipment. The charged particle beam generation equipment includes a synchrotron (or a cyclotron) as an accelerator. A charged particle beam accelerated by the synchrotron up to a level of setting energy reaches the rotating irradiation equipment through the ion beam transport system (i.e., a first ion beam transport system). The rotating irradiation equipment comprises an ion beam transport system within the irradiation equipment (i.e., a second ion beam transport system), irradiation field producing equipment, and a rotating apparatus (rotating gantry) for rotating both the second ion beam transport system and the irradiation field producing equipment in union with each other. After passing the second ion beam transport system, the ion beam is irradiated to a tumor or cancer in the patient body from the irradiation field producing equipment.
The irradiation field producing equipment has the functions of shaping the ion beam from the charged particle beam generation equipment in match with a three-dimensional shape of the tumor, as an irradiation target, to form an irradiation field, and adjusting the dose in the irradiation field. As a method of making irradiation at desired dose in match with the shape of the irradiation target, there is known a double scattering method of producing a uniform dose field by using two types of scatterers, which are arranged in spaced relation in the axial direction of the ion beam, based on a phenomenon that a dose distribution of the ion beam after passing a scatterer becomes substantially a Gaussian distribution (see, e.g., Non-patent Reference 1: “REVIEW OF SCIENTIFIC INSTRUMENTS”, VOLUME 64, NUMBER 8 (AUGUST 1993) P2079-2083).
With the double scattering method, more specifically, the ion beam is first spread into a Gaussian-like distribution by one scatterer (first scatterer) arranged in the upstream side in the direction of travel of the ion beam, and is then adjusted so as to have a uniform does distribution by the other scatterer (second scatterer) arranged in the downstream side in the direction of travel of the ion beam. In medical applications where the ion beam is irradiated to cancer, particularly, it has been strongly demanded as a recent tendency to keep high dose uniformity, to increase the size of the irradiation field to be adaptable for a variety of tumor shapes in the patient bodies, and to increase the penetration depth in the patient body.
In the double scattering method, arranging the scatterers at positions as near as possible to the most upstream side is effective in decreasing the thickness of each of the first and second scatterers to reduce the range loss, and in extending the range to increase the penetration depth in the patient body. While the first scatterer is usually arranged near the most upstream side in the irradiation field producing equipment, the second scatterer is also preferably arranged, from the standpoint of increasing the range length, at a position as near as possible to the most upstream side so that the distance between the first and second scatterers is minimized. On the other hand, in the double scattering method, if a deviation occurs between an axis along which the ion beam travels and the center position of the second scatterer, dose uniformity deteriorates depending on the deviation to a larger extent as the distance between the first and second scatterers decreases. From the standpoint of improving dose uniformity, therefore, the second scatterer is preferably arranged at a position as near as possible to the most downstream side so that the distance between the first and second scatterers is maximized.
As a result, in design of the known irradiation field producing equipment, an optimum mount position of the second scatterer is decided in consideration of balance between a longer range and higher dose uniformity. However, when an available maximum field size is increased in the equipment responsive to the need for a larger size of the irradiation field as mentioned above, a proper mount position of the second scatterer in the case of producing a comparatively large field size greatly differs from the proper mount position thereof in the case of producing a comparatively small field size. Accordingly, it has been difficult to always realize irradiation with a long range and high dose uniformity regardless of the field size.